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PHILADELPHIA — Eight years after Congress passed a law aimed at protecting children from online pornography, free speech advocates and Web site publishers argued in federal court Monday that the never-enforced measure is fatally flawed.

Salon.com, Nerve.com and other plaintiffs warned that the 1998 Child Online Protection Act could be used to criminalize such things as sexual health information, erotic literature and news photographs of naked prisoners tortured at Abu Ghraib.

The law, signed by then-President Clinton, says Web site operators must prevent youngsters from seeing material "harmful to children" by demanding proof of age from computer users. It would impose a $50,000 fine and six-month prison term on commercial Web site operators that allow minors to view such content, which is to be defined by "contemporary community standards."

In a trial that opened Monday, the plaintiffs argued that "community standards" is too vague.

"As a parent, I know that what's fine for my daughter may not be appropriate even for some of her friends," testified Joan Walsh, editor in chief of Salon.com.

The law has never been enforced.

The U.S. Supreme Court has twice upheld preliminary injunctions that prevented the government from enforcing the law until a trial to determine the act's constitutionality can be held.

The American Civil Liberties Union, which represents the plaintiffs, argues that filter programs installed in home computers are more effective ways of policing the Internet.

Eric Beane, a government attorney, acknowledged that it is tempting to defer to families on the question of what is appropriate for children, but said the filters used by parents do not work.

"The evidence will show that a shocking amount of pornography slips through to children," Beane said in opening statements.

ACLU attorney Chris Hansen said that the government is essentially arguing that "parents are too stupid to use filters."

In preparing for its defense of the law, the Justice Department sought internal files from search engine companies and Internet service providers on what sorts of things people search the Internet for.

Google Inc. fought one such subpoena, although it primarily cited trade secrets, not privacy issues. A federal judge in California sharply limited the amount of information Google had to surrender.

The nonjury trial in front of Senior U.S. District Judge Lowell A. Reed Jr. is expected to take about a month.